If yours is like ours, then just above that heater unit is a cabinet that houses our DISH receiver, Bose box, and RF Mogul satellite dish control box... one stacked upon the other. We had to take out all three items (the receiver, Bose box and dish control box) and then remove the wood floor these three items sat upon. Four screws (I think it was) to remove the 1-foot square-ish floor piece, one at each corner of the floor piece, and wallah... the whole top of the heater unit is exposed.
In our case the fan was not operating. After getting that floor piece removed we found a used, cut, black nylon tie had been left in there and the tie had worked its way into the fan to stop it. Removed the tie and it has worked perfectly ever since.